U.S. Pat. No. 3,575,241 issued Apr. 20, 1971 to C. E. McKeon et al for a tractor Hydraulic Lift System disclosing a tractor provided with a torque sensing device in the drive line to the tractor driving wheels to control operation of an implement hydraulic lift system for raising and lowering an implement as required to maintain a constant torque on the drive line. The patented system utilizes a torque sensitive coupling in the driveline which through a complex system of links and levers operates a valve means to raise and lower the implement in response to changes in driveline torque resulting from changes in implement draft load.
It is the object of the present invention to provide a load monitoring system that relies more on hydraulics than mechanical elements for sensing torque changes. This has the advantage that a pre-engineered multi-component valve system replaces a complicated mechanical linkage. It also will be recognized that hydraulic components can be located with considerable flexibility in packaging.